Study On Hot Tearing Formation In Hypoeutectic Al-Si Alloys

Hot tearing, also called hot cracking or solidification cracking, is one of the most serious defects encountered in iron and steel, aluminum alloy and magnesium alloy castings. It is identified as an irreversible defect that appears as cracks or fractures formed during solidification, either on the surface or inside the casting. Most hot tearing appears on the surface can be observed with naked eyes. The microcracks can be observed with the aid of magnetic particle testing, dye penetrant inspection, etc. In practical production, some complicated castings of aluminum alloy are produced. In the solidification of these castings, the thermal stress or tensile stresses are accumulated in the hot spot zone. When the accumulated stresses and strains are greater than the inherent tensile strength of alloys, hot tearing may be initiated and propagated. Once hot tearing occurs, the casting has to be repaired, leading to the immediate productivity loss and the rising cost of production. Therefore, the defect of hot tearing is almost considered to be the most serious problems in casting production and some issues need to be solved further.According to the previous works, many scholars tend to choose the alloys which have a high hot tearing tendency as the research object and they explain the factors of hot tearing formation in alloy composition, cooling rate, mold properties, casting design and process parameters, etc. However, the study on the behavior of hot tearing formation in hypoeutectic Al-Si alloys has not been described. Therefore, the hypoeutectic Al-Si alloys, such as hypoeutectic Al-Si binary alloys and A356alloys, are selected as the reference alloys. The paper aims at simulating the shrinkage behavior of large scale castings and has some research as follows:the solidification behavior for hot tearing formation; cooling rate and axial tensile force influence on hot tearing formation; as-cast microstructure of samples and surface morphology of fracture. And then come to the following conclusions: (1) Based on the quantities of experimental attempts, a new investigated method and Applied Rod Casting Apparatus (ARCA) is developed on our own. This apparatus can meet needs of the study on the alloys which have a low hot tearing tendency. The apparatus has simple structure, reasonable design, which is simple in the follow-up data processing. It can simultaneously collect and record the data of solidification temperature and critical axial tensile forces with high precision and accuracy during the solidification of alloys, providing an important experimental method for further research of solidification behavior and hot tear formation mechanism.(2) Hot tearing is mainly appeared in the late stage of solidification. For the hypoeutectic Al-Si alloys, there are two kinds of mechanism for the hot tearing initiation and propagation. First, stresses apply on the sample in the case of higher temperature. A large amount of residual liquid feeds and heals the interdendritic fracture, keeping the bonding strength between dendrites, so there will be a phenomenon of solidification shrinkage stress or tensile stress relaxation. Along with the solidification of alloys, the reduction of residual liquid phase causes secondary dendrite separation. The initiated hot tearing is propagated under the effect of axial tensile load. Second, stresses apply on the sample in the case of lower temperature. As the solidification of alloys is closed to the stage of eutectic, a large amount of latent heat reduces the cooling rate of solidification, so there is enough time for interdendritic residual liquid phase to feed and heal the initiated crack, increasing the bending strength among dendrites. In brief, it needs larger axial tensile load to induce the hot tearing initiation at this stage.(3) The cooling rate of alloys affects hot tearing formation and a lower cooling rate during solidification can effectively reduce the heat loss in the process of solidification and has a tendency to reduce hot tearing susceptibility of alloys. Increasing the preheating temperature of mold can reduce the interface temperature difference△T’ between of casting and mold, promoting the solidification process of liquid-solid interface to develop steadily into the melt during solidification process. There will not be segregation caused by uneven diffusion of the solute, which can further reduce hot tearing tendency of alloys. On the other hand, if the mold has a lower preheating temperature, it will increase the cooling rate of the melt and also reduce the solute diffusion time. At the same time, it is prone to cause solute segregation of elements in the hot spot, which increases local stress concentration and hot tearing tendency.(4) According to the as-cast microstructure and the fracture surface morphology of hot tearing, it is observed many a-Al dendrites on the fracture surface. The liquid films, intergranular bridge and the liquid folds are coated on the a-Al dendrites. The existing liquid film and intergranular bridge will interrupt the feeding channel, so the initiated microcracks can propagated under the action of axial tensile load or the solidification shrinkage stress. If there are many continuous feeding channels and thickness liquid films, the intergranular separation can be feed and healed by liquid melt. In a short, the liquid film theory and intergranular bridge theory can be consider as the mechanism of hot tearing formation for the hypoeutectic Al-Si alloys. Based on the study, a new hot tearing modeling and criterion is developed. It is defined as a ratio for the nominal yield stress and critical stress for the intergranular separation.